Conventionally, a Mach-Zehnder interferometer is sometimes used in an optical modulator that modulates light generated in a light source. In this optical modulator, a signal electrode and a ground electrode are provided along parallel optical waveguides. In recent years, the optical modulator often includes a plurality of Mach-Zehnder interferometers because optical modulation methods are diversified. In this case, integration of the plurality of Mach-Zehnder interferometers into one chip can reduce a size of the optical modulator.
The optical modulator including the plurality of Mach-Zehnder interferometers can generate a multilevel modulation signal by receiving a plurality of different electric signals input thereto. In other words, the different electric signals are input from an outside to signal electrodes respectively corresponding to the Mach-Zehnder interferometers, so that optical modulation by a multilevel modulation technique, such as a DQPSK (Differential Quadrature Phase Shift Keying) method, can be performed.
In some cases, a unit at which an electric signal is input to the optical modulator and a driver that generates the electric signal to the optical modulator are connected to each other by an flexible printed circuit (FPC) having flexibility.
Specifically, a plurality of wiring patterns are printed on the FPC, which respectively correspond to the plurality of signal electrodes of the optical modulator. The electric signals output from the driver are input to the optical modulator via the wiring patterns printed on the FPC. A driver-side end of the FPC is electrically connected to the driver by being soldered to an electrode pattern to which each wiring pattern outputs the electric signal from the driver, for example. Also, an optical-modulator-side end of the FPC is inserted into a notch formed in a package of the optical modulator, and each wiring pattern is soldered to a coaxial terminal projecting downward from an upper surface of the notch, for example. Due to this configuration, the optical-modulator-side end is electrically connected to the optical modulator.
From a viewpoint of downsizing of a device, a structure is sometimes used in which the optical modulator and the driver are hierarchically arranged by using different substrates and are connected by the FPC.
However, in the structure in which the optical modulator and the driver hierarchically arranged are connected by the FPC, an arrangement space for the optical modulator and an arrangement space for the driver are separated from each other. This structure may increase a mounting area of the entire device. Therefore, the structure in which the optical modulator and the driver are hierarchically arranged is not practical.
It can be considered that a portion of the driver is accommodated in the notch formed in the package of the optical modulator to reduce a mounting area corresponding to the driver. However, in this case, the coaxial terminal projecting into the notch and the driver are arranged to be close to each other, causing sharp bending of the FPC that connects the coaxial terminal and the electrode pattern extending from the driver. The bending of the FPC causes increase in the length of the FPC in order to absorb the bending.
The electric signal supplied from the driver to the optical modulator via the FPC is a high-frequency signal. Therefore, in a case where the electric signal is transmitted by the FPC, it is known that attenuation of the electric signal increases as the FPC becomes longer. In other words, in a case where the portion of the driver is accommodated in the notch formed in the package of the optical modulator, when the optical modulator and the driver are connected by using the FPC, the mounting area is reduced. However, there is a problem that high-frequency characteristics of the electric signal supplied to the optical modulator are deteriorated.